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Abstract
A novel compact ultra-high sensitivity optical fiber temperature sensor based on surface plasmon resonance (SPR) is proposed and demonstrated. The sensor is fabricated by employing a helical-core fiber (HCF), which is polished as a D-type fiber on the helical-core region and coated with a layer of Au-film and polydimethylsiloxane (PDMS). The theoretical and experimental results show that the resonant wavelength and sensitivity of the proposed sensor can be effectively adjusted by changing the twisting pitch of HCF. Due to the high refractive index sensitivity of the sensor and the high thermo-optic coefficient of PDMS, the maximum sensitivity can reach -19.56 nm/°C at room temperature when the twist pitch of HCF is 2.1 mm. It is worth noting that the sensitivity can be further improved by using a shorter pitch of HCF. The proposed SPR temperature sensor has adjustable sensitivity, is easy to realize distributed sensing, and has potential application prospects in biomedical, healthcare, and other fields.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Accurate temperature measurement and control are very important in biomedicine, chemical reaction, environmental monitoring, and other applications. Compared with traditional electronic sensors to measure temperature, optical fiber sensors have many advantages, such as high sensitivity, anti-electromagnetic interference, and compact structure, which has attracted wide attention in recent years [1,2]. Up to now, many types of optical fiber temperature sensors have been proposed and extensively studied, such as fiber Bragg gratings (FBGs) [3,4], Fabry-Perot interferometers (FPIs) [5–7], Mach-Zehnder interferometers (MZIs) [8–10], and Sagnac interferometers [11], which have good performance in temperature sensing applications, but their sensitivity is low (pm/℃ level). Other schemes, such as whisper gallery modes (WGMs) [12], micro-nano fiber [13], and photonic crystal fiber (PCF) sensors [14,15], have the characteristics of high sensitivity, but their stability and repeatability are poor and their working range is limited.
Surface plasmon resonance (SPR) is a physical phenomenon of the optic excitation of surface plasmon wave (SPW) at the interface between a metal layer and a dielectric layer [16]. Because of the high sensitivity to the variation of the refractive index (RI), SPR technology based on the bulk-optic and fiber-optic systems has been widely applied in chemical, biological, and environmental monitoring [17]. However, fiber-optic SPR devices have more advantages than bulk-optic SPR devices in miniaturization and integration, remote sensing, real-time and in situ monitoring [18–21]. Considering the natural thermo-optical effect of the sensing materials, several kinds of optical fibers are applied in temperature SPR sensing, including single-mode fiber [22–24], multi-mode fiber [25], photonic crystal fiber [26], hollow-core fiber [27], and no-core fiber [28]. Recently, Srivastava et al. proposed and analyzed a fiber optic temperature sensor utilizing localized surface plasmon resonance (LSPR) of gold nanoparticles embedded in CdGeP2 as a temperature sensing tool [29]. Wang et al. designed and fabricated a novel optical fiber SPR sensor based on multimode fiber-photonic crystal fiber- multimode fiber (MMF-PCF-MMF) structure coated with a gold and polydimethylsiloxane (PDMS) film, and achieved a temperature sensitivity of -1.551 nm/℃ [30]. Siyu et al. filled the hole of a hollow-core fiber with PDMS to realize a temperature sensitivity of -1.05 nm/℃ [31]. Luo et al. coated graphene and PDMS onto an optical-fiber-based plasmonic interface to realize a quick-response temperature SPR senor with the sensitivity of 1.56 nm/℃ [32]. Yang et al. realized an ultrawide temperature range operating of SPR sensor utilizing a depressing double cladding fiber coated with Au-PDMS. In the temperature range from -30 to 330°C, the temperature sensitivity of the sensor can reach -2.27 nm/°C [33]. Lu et al. proposed and demonstrated a portable optical fiber SPR temperature sensing platform based on a smartphone, which has a sensitivity of -0.0018 a.u./℃ for the temperature ranging from 30 to 70℃ [34]. The optical fiber SPR sensor has unique advantages in temperature measurement, such as high sensitivity, small size, anti-electromagnetic interference and so on. However, it is difficult to adjust the resonance wavelength and sensitivity due to the limitation of the optical fiber structure.
In our previous work [35], we proposed and demonstrated a novel and flexible SPR sensor based on a side-polishing HCF. The resonance wavelength and sensitivity of the HCF SPR sensor can be effectively adjusted by change the twist pitch of HCF. Additionally, we cascaded two HCF SPR sensors with different pitches to achieve multi-parameter measurement [28]. Compared with general bending fiber SPR sensors [36,37], this sensor does not need to be packaged in a substrate, which makes it compact and flexible. In this paper, an ultra-high sensitivity SPR temperature sensor based on helical-core fiber coated with PDMS is proposed and demonstrated. The proposed sensor with a twist pitch of 2.1 mm realizes a high sensitivity of 19.56 nm/°C to 4.79 nm/°C in the range from 20°C to 70°C, which is almost one order of magnitude higher than that of other types of optical fiber temperature sensors. This sensitivity can be further improved by using a shorter pitch HCF. Because of its portability, miniaturization and easy integration, the sensor has great application potential in biomedicine, health care and other fields.
2. Operating principle and fabrication
The schematic diagram of the proposed HCF SPR temperature sensor is depicted in Fig. 1(a). The sensor is fabricated by a side-polished HCF, coated with a 50 nm thick gold film and then coated with PDMS. The HCF is manufactured by twisting an eccentric core fiber (ECF), whose section image and RI distribution are shown in Figs. 1(c) and 1(d), respectively. The HCF has only one helical period as shown in Fig. 1(b). And the coating length is longer than the twist pitch of HCF.
Fig. 1. Helical-core fiber SPR temperature sensor. (a) Working principle of SPR sensing. The nearest distance between the helical-core and side-polishing surface is d0 = 4.5μm. (b) Schematic diagram of HCF. P is the twisting pitch. (c) The cross-section of HCF. The core offset Q is 28 μm. (d) RI distribution of HCF. The numerical aperture of the fiber is 0.08. (e) and (f) are normalized mode field distributions of p-polarized fundamental core modes propagation at 820 nm resonant wavelength in HCF SPR sensor with same pitch of 5.0 mm but different d0 = 4.5μm and 0.5μm, respectively.
For simplicity, the working principle of the HCF SPR sensor can be divided into three steps, which correspond to stage I, stage II and stage III shown in Fig. 1(a), respectively. In the first stage, a wideband light is coupled into the straight input core of the lead-in ECF, and a guided-core mode is generated. Due to the bending effect of the helical core, partially guided core mode will leak out of the helical core and form cladding WGMs when the core mode propagates along with the helical core [38]. In the second stage, the WGMs will propagate in the cladding and can easily reach the side-polishing area. The evanescent wave of the cladding WGMs can excite the SPW at the boundary between the gold layer and surrounding medium when propagation constants between WGMs and the surface plasmon mode (SPM) are matched. In the third stage, after the coupling among the core mode, cladding WGMs, and SPMs, part of cladding WGMs can be coupled to the straight output core of ECF, forming a core guiding mode. It should be noted that only part of the WGMs, whose mode distribution in the core area is similar to that in the straight core mode, can be effectively coupled into the output core. The WGMs are closely related to the twisting pitch, so we can effectively control the SPR resonance effect by changing the twisting pitch of helical core fiber.
For simplicity, based on the mode expansion and propagation (MEP) method, an equivalent 2D bending waveguide model is introduced to analyze wave propagation in the HCF SPR sensor [35]. The bending radius of the equivalent 2D bending core of the HCF can be defined as
where P and Q are the twist pitch and offset of the helical core (see Fig. 1(b)), respectively. The HCF cladding refractive index as a function of wavelength can be described by the Sellmeier’s equationThe fabrication of the HCF SPR temperature sensor can be divide into four steps: fiber twisting, side-polishing, depositing a gold film, and coating PDMS. Firstly, ECF is twisted and translated at the same time by using the heating process of high voltage arc discharge. By controlling the speeds of twisting and translating, we can obtain the HCF sample with proper pitch, as shown in Fig. 2(a). Then the side-polishing technique is used to polish the HCF and partially remove the cladding of HCF. In the side polishing process, we roughly polished the HCF for about 10 minutes and then finely polished it for ∼2 hours to obtain the desired residual cladding thickness. The side-polished area was immersed in the matching liquid and monitored by a microscopic imaging system, as shown in Figs. 2(b) and 2(c). Then, a plasma sputtering apparatus (JS-1600, HTCY) was introduced to deposit gold film (∼50nm) on the side-polishing surface of HCF. Besides, the thickness of the gold film was measured by the three-dimensional morphology analyzer (NewView 7200, Zygo), as shown in Figs. 2(d) and 2(e), the depth of the groove represents the thickness of the gold film. After gold film deposition, we mixed the liquid PDMS and curing agent in a mass ratio of 10: 1, and then stirred the mixture with a vacuum mixer until all the bubbles were completely removed. Then, we coated the side-polished surface of HCF with PDMS mixture and heated it at 60℃ for 2 hours. Finally, the PDMS mixture solution will be completely cured, and the HCF SPR temperature sensor can be obtained.
Fig. 2. Twisting and side-polishing process. (a) Helical-core fiber with a twist pitch of 2.5 mm. Multiple pictures along the HCF were taken and stitched together. (b) and (c) are side views of polished fiber with different side polishing depths. (d) and (e) are three-dimension and one-dimension gold film measured results of the coating gold film at the side-polishing region.
3. Experimental setup
The high thermal optical coefficient (TOC) of PDMS plays an important role in the proposed temperature sensor. In order to optimize the sensor, the refractive index of PDMS needs to be measured. Put the proposed PDMS mixture solution into a petri dish to form a very thin slice, and heat at 60℃ for 2 hours until the PDMS slice was completely cured. Then, we cut the PDMS slice into small pieces of 1 cm × 1 cm, and measured their refractive index at different temperatures with Abbe refractometer (GDA-2S, from Gold). The measurement results within a temperature range of 20-70 degrees are shown in Fig. 3. We can find that the RI of PDMS decreases when the temperature rises from 20℃ to 70℃. The relationship between PDMS refractive index (nPDMS) and temperature (T) can be written as follows by linear fitting:
where the slope -4.5×10−4 represents the TOC of PDMS. It can be seen from Fig. 1(a) that the proposed HCF SPR temperature sensor is coated by PDMS, so the sensor realizes temperature sensing by measuring the change of refractive index of PDMS with temperature.The experimental setup for temperature measurement utilizing the HCF SPR temperature sensor is shown schematically in Fig. 4. The HCF SPR sensor is immersed in a water bath, and the temperature of which is controlled by a temperature controller. The temperature range is 0-100℃, and the temperature precision is 0.1℃. The transmission spectra were measured by coupling the broadband light source (SuperK compact, from NKT Photonics) to the HCF SPR sensor and recording the received with a spectrometer (AQ 6370, from Yokokawa). As shown in the inset of Fig. 4, by using a common manual mode fiber splicer and monitoring the output power, the input and output ends of ECF are spliced to SMF through core alignment.
Fig. 4. Schematic of the proposed experimental setup of HCF SPR temperature sensing. The upper inset shows a photograph of the HCF SPR temperature sensor coating PDMS. The under-upper inset is fusion splicing between the SMF and the ECF.
4. Results and discussion
In the manufacturing process of HCF SPR temperature sensor, the optical loss can be introduced by fusion splicing between the SMF and the ECF, twisting the ECF, side polishing and gold film coating, as shown in Fig. 5(a). However, before the sensor was coated with PDMS, we could not observe the resonance dip angle indicating SPR excitation during the manufacturing process of the sensor, as shown in the curve labeled E in Fig. 5(a). In order to maintain the SPR loss and eliminate other losses, the output transmission spectra of the HCF SPR temperature sensor is processed by using the difference between output transmission spectra measured without PDMS coating and with PDMS coating in different temperature. Then, the output transmission spectra are normalized by its maximum and minimum values. Figure 5(b) shows the output spectra of the HCF SPR temperature sensors of different twist pitch measured at 20℃.
Fig. 5. The experiment results of the HCF SPR temperature sensor in the process of fabrication and measurement. (a) Output spectra (marked A-E) after light source input, fusion splicing between the SMF and ECF, twisting the ECF to form HCF with a pitch of 2.5 mm, side-polishing without and with PDMS coating, respectively. (b) Normallized output spectra of the HCF SPR temperature sensor measured in temperature of 20℃ for different twist pitches of HCF from 2.1 mm to 5.0 mm.
To compare the effects of different twist pitches on the performance of the proposed HCF SPR temperature sensor, we fabricated sensors with pitches of 2.1 mm, 2.5 mm, 3.2 mm, and 5.0 mm for experiments. The experimental results of output spectra are shown in Figs. 6(a)–6(d). By using the transfer matrix method [35], we also calculated the normalized transmission spectrum of the p-polarized multi-WGMs transmission in the HCF SPR sensing region claddings, as shown in Figs. 6(e)–6(h). And the corresponding experimental and theoretical results of resonance wavelength changing are given by Fig. 7(a). Except for the depth of the resonance spectra, we can find that the experimental results are in good agreement with the theoretical calculation. Therefore, the cladding WGMs have good performance in studying the change of resonance wavelength changing (see Fig. 7(a)), but the performance in analyzing the depth of the resonance spectrum is poor. With the increase of temperature, the refractive index of PDMS decreases, and the resonance wavelength shifts to shorter wavelengths. Besides, the smaller the pitch of the HCF, the more obvious the tendency of the resonance wavelength changing with temperature. In other words, the sensing sensitivity increases as the pitch of HCF decreases.
Fig. 6. Normalized experimental spectra measured in temperature varied from 20 ℃ to 70 ℃ for different twist pitch. (a) P = 2.1 mm. (b) P = 2.5 mm. (c) P = 3.2 mm. (d) P = 5.0 mm. (e)-(h) are corresponding calculation results.
Fig. 7. The sensing characteristics of HCF SPR temperature sensor. (a) The resonant wavelength shifts with the measurement temperature. The scatter and solid lines represent experimental and theoretical results, respectively. (b) Average sensitivity curves.
For a short pitch, the resonant wavelength decreases rapidly with the increase of temperature, showing a quadratic function relationship, as shown in Fig. 7(a). Sensitivity also decreases with the increase of temperature, but it presents a quasi-linear function relationship, as shown in Fig. 7(b). By comparing the results of different twisting pitches, we find that the resonant wavelength range and the sensing sensitivity increase with the decrease of the pitch in the same temperature range. In our case, the resonant wavelength range of sensors with twisting pitched of 5.0 mm and 2.1 mm are 745.6-927.0 nm and 904.2-1450.2 nm in a temperature range of 20-70℃, respectively. For the sensor with a twisting pitch of 2.1 mm, the highest sensitivity reaches 19.56 nm/℃ at 20℃. It is worth mentioning that by using a shorter twisting pitch HCF, the sensitivity can be further improved. In addition, we also test the stability of the proposed HCF SPR temperature sensors, as shown in Fig. 8. We recorded the transmission spectrum per minute and raised the temperature by 5℃ per 10 min. As seen in Fig. 8, the stability of the proposed HCF SPR temperature sensor is very good.
Table 1 shows the comparison of temperature sensitivity and temperature range of several fiber-based SPR temperature sensors. The sensitivity of the HCF SPR with a twisting pitch of 2.1 mm reaches 19.56-4.79 nm/℃ in the temperature range of 20-70℃. And we can obtain the maximum sensitivity of 19.56 nm/℃ at 20℃, which is almost one order of magnitude and twice higher than that of other types of sensors at low and high temperature sensing, respectively.
Table 1. Comparison of fiber-based SPR sensors for temperature sensing from experimental studies
5. Conclusion
In conclusion, a novel optical fiber SPR temperature sensor has been proposed and demonstrated based on helical-core fiber coated with gold film and PDMS. The influence of twisting pitch on sensing performance is analyzed experimentally and theoretically. The resonant wavelength and sensing sensitivity increase as the decreases of twisting pitch. For short twisting pitch, such as 2.1 mm, the proposed sensor shows an ultra-high sensitivity, which can be nearly one order of magnitude higher than the general fiber-based SPR sensors at room temperature. And the stability of the sensor is also excellent. By changing the twisting pitch, we can obtain sensors with different working ranges and sensitivities to meet different applications. The proposed HCF SPR temperature sensor has widespread application in many fields, including biomedical, biomaterial, and environmental monitoring.
Funding
National Key Research and Development Program of China (2017YFB0405501); National Natural Science Foundation of China (61675052, 61705050, 61827819, 61965005); Guangxi Project (AD18281045).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are available in Refs. [35] and [38].
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